Modified zeolite omega and processes for preparing and using same

ABSTRACT

Forms of zeolite Omega synthesized by hydrothermal crystallization from reaction systems containing alkali metal cations and organic templating agents, modified by calcination in air, ion-exchange, steam calcination and treatment with a low-pH aqueous ammonium ion solution, are significantly improved with respect to surface area, catalytic activity and adsorption capacities for large molecular species.

This application is a continuation of prior U.S. application Ser. No.628,830, filed Dec. 17, 1990.

FIELD OF THE INVENTION

The present invention relates in general to a modified form of zeoliteOmega and, more particularly, to a form of zeolite Omega having anexceptionally high surface area, adsorption capacity for large moleculesand improved catalytic activity. The starting zeolite Omega is calcinedin air, ammonium ion exchanged, steamed at a temperature of at least550° C. and then again subjected to an ammonium ion exchange using anexchange medium having a pH of less than 4.0, preferably in the range ofabout 0.5 to about 4.0. The invention also relates to the use of themodified zeolite compositions in catlytic hydrocarbon conversionreactions.

BACKGROUND OF THE INVENTION

Zeolite Omega was first synthesized more than fifteen years ago. Thesynthesis techniques and characterization of the synthetic zeolite arereported in U.S. Pat. No. 4,241,036 issued Dec. 23, 1980, to E. M.Flanigen et al, the entire disclosure of which is incorporated byreference herein. Other synthesis processes have subsequently beendeveloped in which the organic templating agent employed is a differentorganic amine, namely, (β-hydroxyethyl) trimethylammonium hydroxide(choline), choline chloride, pyrrolidone, or 1,4-diazobicyclo (2.2.2)octane (DABCO), the latter also called triethylenediamine (TED). Theseprocesses are disclosed, respectively, in U.S. Pat. Nos. 4,021,447 and4,331,643, both issued to Rubin et al. See also U.S. Pat. No. 4,377,502for additional synthesis procedures. Zeolite Omega is typicallycrystallized hydrothermally from a reaction mixture having a compositionexpressed in terms of mole ratios of oxides within the ranges

    ______________________________________                                        (Na.sub.2 O + R.sub.2 O)/SiO.sub.2                                                              from about 0.1 to about 0.6                                 R.sub.2 O/(R.sub.2 O + Na.sub.2 O)                                                              from >0 to about 0.6                                        SiO.sub.2 /Al.sub.2 O.sub.3                                                                     from about 5 to about 30                                    H.sub.2 O(R.sub.2 O + Na.sub.2 O)                                                               from about 10 to about 125                                  ______________________________________                                    

wherein "R" represents the tetramethylammonium or other organic cation.Crystallization periods of from about 1 to 8 days at temperatures offrom 90° C. to 180° C. are usually satisfactory. The as-synthesizedzeolite Omega typically has a chemical composition (anhydrous basis) interms of molar oxide ratios of

    (xR.sub.2 O+yNa.sub.2 O):Al.sub.2 O.sub.3 :5to 20 SiO.sub.2

wherein (x+y) has a value of from about 1.0 to 1.5 and x/y having avalue usually in the range of about 0.35 to 0.60.

In addition to composition and in conjunction therewith, zeolite Ω canbe identified and distinguished from other crystalline substances by itsX-ray powder diffraction pattern, the data for which are set forth belowin Table A. In obtaining the X-ray powder diffraction pattern, standardtechniques were employed. The radiation was the K₆₀ doublet of copper,and a Geiger counter spectrometer with a strip chart pen recorder wasused. The peak heights, I, and the positions as a function of 2Θ, whereΘ is the Bragg angle, were read from the spectrometer chart. From these,the relative intensities, and d(Å) observed, the interplanar pacing inAngstrom units corresponding to the recorded lines were determined. InTable A, the more significant interplanar spacings, i.e., the d(Å)values which characterize and distinguish zeolite Ω from other zeolitespecies and which must be present in the X-ray powder diffractionpattern of the zeolite Ω composition of the present invention, are setforth. The relative intensities of the lines are expressed as VS (verystrong), S (strong), MS (medium strong) and M (medium).

                  TABLE A                                                         ______________________________________                                        d, (A)       Relative Intensity                                               ______________________________________                                        9.1 ± 0.2 VS                                                               7.9 ± 0.2 M                                                                6.9 ± 0.2 M-S                                                              5.95 ± 0.1                                                                              M-S                                                              4.69 ± 0.1                                                                              M-S                                                              3.79 ± 0.1                                                                              S                                                                3.62 ± 0.05                                                                             M-S                                                              3.51 ± 0.05                                                                             M-S                                                              3.14 ± 0.05                                                                             M-S                                                              3.08 ± 0.05                                                                             M                                                                3.03 ± 0.05                                                                             M                                                                2.92 ± 0.05                                                                             M-S                                                              ______________________________________                                    

Thus, zeolite Ω can be defined as a synthetic cyrstallinealuminosilicate having an X-ray powder diffraction pattern characterizedby at least those interplanar spacing values set forth in Table A andhaving the stoichiometric compositions as set forth hereinbefore. TheX-ray data given below in Table B are for a typical example of zeolite Ωprepared in the sodium, TMA system.

                  TABLE B                                                         ______________________________________                                        d, (A)              Intensity                                                 ______________________________________                                        15.95                   20                                                    9.09                    86                                                    7.87                    21                                                    6.86                    27                                                    5.94                    32                                                    5.47                    6                                                      5.25                                                                                           *     8                                                     5.19                                                                          4.695                   32                                                    3.909                   11                                                    3.794                   58                                                    3.708                   30                                                    3.620                   25                                                    3.516                   53                                                    3.456                   20                                                    3.13                    38                                                     3.074                                                                                          *     21                                                    3.02                                                                          2.911                   36                                                    2.640                   6                                                     2.488                   6                                                     2.342                   17                                                    2.272                   6                                                     2.139                   5                                                     2.031                   17                                                    1.978                   5                                                     1.909                   10                                                    1.748                   6                                                     ______________________________________                                         * = doublet                                                              

As synthesized, the crystallites of zeolite Omega are usually quitesmall and are recovered as anhedral to spherical growth agglomeratesfrom about 0.2 to several microns in size. The acid stability of thecrystals is relatively high, producing a buffering effect at a value ofabout 1.6 when titrated with a 0.25N aqueous solution of HCl. By thistest zeolite Omega is more acid-stable than zeolite Y, less acid stablethan mordenite, and essentially the same as the natural zeoliteserionite, clinoptilolite and chabazite.

The pore diameters of the zeolite are quite large, at least about 8Angstroms, as evidenced by the adsorption of more than 15 weight percentof (C₄ F₉)₃ N at 50° C. and a pressure of 0.7 mm. Hg in each of thesodium, calcium, potassium and ammonium cation forms after calcinationto remove the organic ions and/or compounds from the internal cavities.The organic species are not capable of being removed as such through thepore system, because they are intercalated in the structural gmelinitecages which are arranged in the crystal lattice to form the large poresalong the crystallographic "c" axis.

The basic chemical and physical properties of zeolite Omega as indicatedby the aforementioned evaluations suggest, a priori, that it would haveconsiderable commercial capabilities as a catalyst or catalyst base inmany of the hydrocarbon conversion processes which employ otherlarge-pore zeolites such as zeolite Y. This potential has not beenrealized, however, due in large part to an apparent lack of consistencyin the acidic and adsorptive properties observed in different synthesisbatches of the zeolite.

A particularly important and significantly variable property is thethermal stability of the as-synthesized zeolite Omega. As reported byWeeks et al in JCS Farad. Trans. 1, 72 (1976), zeolite Omega in thesodium or ammonium cation form is either destroyed or undergoes aconsiderable decrease in crystallinity by calcination in air at 600° C.,a phenomenon attributed by the authors to the loss of TMA⁺ ions at aboutthat temperature. Others have suggested that the small crystallite sizeof the analyzed samples is, in part at least, responsible for therelatively low thermal stability. On the other hand, in U.S. Pat. No.4,241,036, Flanigen et al report that zeolite Omega is stable up toabout 800° C. when heated in air or vacuum and that when heated for 17hours at temperatures within the range of 300° C. to 750° C., thezeolite undergoes no appreciable loss in X-ray crystallinity, but thatat 400° C. there is an appreciable loss of TMA⁺ cations by thermaldecomposition. It would appear from the foregoing that one or moreaspects of the synthesis procedure, as yet unidentified, can have amarked effect upon the physical and/or chemical properties of zeolite Ω,and that these differences account, at least in part, for the somewhaterratic catalytic properties noted by prior investigators in zeolite Ωcompositions which, by virtue of their provenance, would be expected tobe nearly identical.

In U.S. Pat. No. 4,780,436, issued Oct. 25, 1988, to Raatz et al, thischaracteristic instability of zeolite Ω is discussed and a stabilizationand dealumination procedure is proposed which converts theas-synthesized form of the zeolite to a form with more reproduciblecatalytic behavior. The treatment proposed by Raatz et al is athree-step procedure which comprises:

(a) a first step of subjecting the synthetic zeolite to a treatment forremoving the major part of the TMA⁺ cations, while decreasing the alkalimetal cations to less than 0.5 percent by weight;

(b) a second step of subjecting the product of the first step to atleast one calcination in air, steam or a mixture of air and steam at atemperature of from 400° C. to 900° C.; and

(c) acid etching the product of the second step with an inorganic acidsuch as HCl or an organic acid such as acetic acid.

The procedure is alleged to cause some shrinkage of the unit cellconstants, a_(o) and c_(o) to below 1.814A and 0.759A, respectively, toincrease the nitrogen adsorption capacity and to create a mesoporestructure which corresponds to about 5 to 50 percent of the combinedmesopore and micropore volume. The catalytic acidity of the zeolite isalso said to be improved.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a novelmodified form of zeolite Omega having adsorption properties andcatalytic activity significantly different from those possessed by anyof the known prior art forms of zeolite Omega. The method for preparingthese unique compositions is also novel and is a part of the invention.

The zeolite composition of the invention has a chemical composition interms of molar oxide ratios in the anhydrous state of

    xM.sub.2/n O:Al.sub.2 O.sub.3 :ySiO.sub.2

wherein "M" represents a cation having the valence "n," "x" has a valueof from zero to about 1.2, "y" has a value of at least 7, preferablygreater than 10, an X-ray diffraction pattern containing at least thefollowing d-spacings

    ______________________________________                                        d, (A)       Relative Intensity                                               ______________________________________                                        9.1 ± 0.2 VS                                                               7.9 ± 0.2 M                                                                6.9 ± 0.2 M-S                                                              5.95 ± 0.1                                                                              M-S                                                              4.69 ± 0.1                                                                              M-S                                                              3.79 ± 0.1                                                                              S                                                                3.62 ± 0.05                                                                             M-S                                                              3.51 ± 0.05                                                                             M-MS                                                             3.14 ± 0.05                                                                             M-S                                                              3.08 ± 0.05                                                                             M                                                                3.03 ± 0.05                                                                             M                                                                2.92 ± 0.05                                                                             M-S                                                              ______________________________________                                    

an adsorption capacity for SF₆ of at least 6.0 weight percent whenmeasured at 22° C. and an SF₆ pressure of 400 mm. Hg, an adsorptioncapacity for oxygen of at least 20 weight percent when measured at -183°C. and an oxygen pressure of 100 mm. Hg, and a surface area (B.E.T.) ofat least 500 m² /gram. The zeolite composition preferably has a butaneCracking Activity, K_(a), of at least 160.

In preparing the novel composition, the procedure comprises the steps of

(a) providing an as-synthesized zeolite Omega starting materialcontaining alkali metal and organic cations, calcining the startingcomposition, preferably in air at a temperature in the range of about400° C. to 600° C. to thermally decompose the organic cations;

(b) contacting the calcined product of step (a) with an aqueous solutionof non-metallic cations under cation exchange conditions to lower thealkali metal cation content to below 0.1 equivalent percent;

(c) calcining the ion-exchanged product of step (b) in contact with atleast 3 psia steam at a temperature of from about 400° C. to 800° C.,preferably from 500° C. to 575° C., preferably for a period of at leastabout 2 hours; and thereafter

(d) contacting the steamed product of step (c) with a sufficient amountof an aqueous solution of ammonium ions having a pH of less than about4.0 and for a sufficient time to increase the bulk Si/Al₂ ratio of thezeolite composition with respect to the starting composition of step (a)and to a value of at least 7.0.

DETAILED DESCRIPTION OF THE INVENTION

The starting zeolite used to prepare the novel compositions of thepresent invention can be any zeolite Omega prepared in accordance withthe teachings of the prior art such as the Flanigen et al U.S. Pat. No.4,241,036. The zeolite is sometimes denominated as ZSM-4, and istopologically related to the mineral mazzite. Another synthetic zeolitespecies knows as LZ-202 is also topologically related to zeolite Omega,but is prepared in the absence of an organic templating agent. See U.S.Pat. No. 4,840,779 issued to T. R. Cannan in this regard. Since theinherent thermal instability of activated zeolite Omega is thought to berelated to initial presence and subsequent removal of the organiccations, neither mazzite nor LZ-202 is believed to benefit from thetreatment in accordance with the present invention. Of the startingzeolite Omega compositions containing both alkali metal and organiccations, those in which the framework Si/Al₂ molar ratio is within therange of 5-12 are preferably employed.

In the initial step of the preparative process the removal of the TMA⁺and/or other organic templating cations is accomplished by calcinationat a temperature of at least 400° C., and preferably between 500° C. and600° C. While essentially any inert atmosphere can be used in thecalcinations, i.e., nitrogen, helium, hydrogen and the like, it isadvantageous to utilize an atmosphere containing oxygen in sufficientamount to convert the non-volatile carbon residue of the decomposedorganic template to CO or CO₂ so that it can be readily removed from thezeolite pore system. Air is a convenient oxidative medium either per seor supplemented with additional oxygen. Water vapor need not be entirelyexcluded from the calcination atmosphere, but should be limited topartial pressures of less than about 1.5 psia.

Following the calcination to remove the organic cations, the zeolite ision-exchanged with non-metallic cations to reduce the alkali metalcontent of the zeolite to less than about 0.1 equivalent percent. Theterm non-metallic cations is intended to mean hydrogen or ammoniumcations or precursors of hydrogen or ammonium cations. It is highlypreferred that the cations substituted for the alkali metal cationsconsist of, or at least comprise, ammonium cations. In general, thenon-metallic nitrogen-containing cations which are precursors ofhydrogen or ammonium cations, such as the tetraalkylammonium and otherquaternary ammonium compounds, are relatively large ionic species whichhave difficulty in rapidly diffusing through the pore system of thezeolite to contact the alkali metal cations. In addition, these organicspecies are in general quite expensive and their use needlesslyincreases the costs of the process. Hydrogen cations introduced byion-exchange with an inorganic or organic acid medium are entirelysuitable for the practice of the preparative procedure, but it issometimes difficult to obtain the necessary replacement of alkali metalcations without damage to the zeolite crystal structure. Accordingly,the ion exchange is carried out in any manner conventional in the art,preferably with an aqueous solution of an ammonium salt such as NH₄ Cl,NH₄ NO₃ or (NH₄)₂ SO₄ at a temperature of from about 25° C. to 100° C.,preferably about 90° C. Advantageously multiple-step procedures are usedin which the zeolite is contacted with a series of fresh ion-exchangesolutions which prevents the creation of an equilibrium condition fromdeveloping as exchanged alkali metal ions from the zeolite become moreconcentrated in the exchange solution. After the alkali metal cationcontent of the zeolite has been reduced to below 0.1 weight percent, thezeolite is washed with water to remove any occluded salt.

The low-alkali metal zeolite is then steamed at a temperature of from500° C. to 900° C., preferably from 550° C.-750° C., for a period ofabout 0.5 to about 2 hours, depending somewhat upon the temperature,with at least 3.0 psia steam, preferably 100% steam. The steamingprocedure appears to remove aluminum atoms from the crystal lattice, butthe mechanism has not been fully elucidated. The available literature onthe subject indicates that the dealumination of zeolite Omega does notclosely parallel the much more thoroughly investigated dealuminationmechanism of zeolite Y using steam. Moreover, the experimental datapresented hereinafter provide added evidence that the dealuminations ofzeolite Omega and zeolite Y do not occur in the same or similar manner.Regardless of the mechanism, the steaming should be continued untilthere is at least some reduction in the a_(o) unit cell constant and,preferably, to not more than about 18.21 Angstroms.

The steamed zeolite product is then contacted with a low-pH aqueoussolution of ammonium ions. The concentration of the ammonium ionsolution is not a critical factor, but is generally in the range of 100to 300 gram ions/liter of NH₄ ⁺. The amount of ammonium ion solutionrelative to the zeolite composition is also not critical, but solutionscontaining from 100 to 200 gram ions of NH₄ ⁺ per 100 grams of zeolite(anhydrous basis) have been found to be suitable. As in the case of theion-exchange of step (b) of the preparative process, supra, multiple,preferably three, contacts of the zeolite with fresh solutions ofammonium ions are more effective than one step treatments. The ammoniumions can be provided by any of the common ammonium salts such as NH₄ Cland NH₄ NO₃, the latter being preferred. It is a critical matter thatthe pH of the NH₄ ⁺ ion solution be not greater than 4.0 and ispreferably in the range of 3.0 to 4.0. The pH can readily be adjusted tothe proper range by the addition of a mineral acid such as nitric orhydrochloric acid. The temperature of contact of the zeolite and the NH₄⁺ solution is generally in the range of 25° C. to 100° C., preferablyabout 90° C. Optimum conditions of contact time, temperature andconcentration of ammonium ions are readily determined for each zeoliteOmega starting material by periodic monitoring the physical and chemicalproperties of the zeolite.

The process of preparation and the unique properties of the resultingmodified zeolite Omega are illustrated by the following examples. Inmaking the surface area, adsorption capacity and Butane CrackingActivity measurements referred to in the Examples, the followingprocedures were used:

(a) Surface Area -- Determined utilizing the Brunauer-Emmett-Teller(BET) theory of multilayer adsorption. The surface area is determined bymeasuring the volume of nitrogen gas adsorbed at liquid nitrogentemperatures. The single point analysis is used. Sample preparation isaccomplished by heating the sample to 400° C. and evacuating to apressure of less than 10 μm for 16 hours.

The surface area is calculated from the experimental data according tothe equation:

    Surface area (m.sup.2 /g=4.35(1/S+I)

wherein ##EQU1## In single point analyses, the value of I is zero sincethe intercept passes through the origin. V_(a) represents the volume ofnitrogen adsorbed. The BET surface area determination is well known inthe art.

(b) SF₆ Adsorption Capacity -- a conventional McBain-Bakr adsorptionapparatus was employed. The test sample was activated at 400° C.overnight under vacuum of 10⁻⁵ torr and then cooled to 22° C. SF₆ wasintroduced into the apparatus in contact with the zeolite sample at apressure of 400 mm. Hg and at a temperature of 22° C. The weightdifference between the starting zeolite and the zeolite in contact withthe SF₆ after 2 hours was calculated and reported in terms of weightpercent, anhydrous basis of the zeolite. The anhydrous weight of zeoliteis determined after calcination at 400° C. for 16 hours.

(c) Oxygen Adsorption Capacity -- Determined in the same manner as inthe case of SF₆, supra, except the oxygen pressure was 100 mm Hg and thetemperature was -183° C.

(d) Butane Cracking Activity -- The procedure described in detail by H.Rastelli et al in the Canadian Journal of Chemical Engineering, 60, pgs.44-49 (1982), incorporated by reference herein, was employed.

EXAMPLE 1 Synthesis of zeolite Omega 13890-39

A zeolite Ω sample was prepared hydrothermally from a reaction mixturehaving the following composition in terms of mole ratios of oxides:

    8SiO.sub.2 :Al.sub.2 O.sub.3 :0.25(TMA).sub.2 O:5Na.sub.2 O:140H.sub.2 O

In the preparation of the reaction mixture 15.7 pounds of flake NaOH(98%) were dissolved in 34.4 pounds of water. To this solution wereadded simultaneously 127.4 pounds of sodium silicate (29.1 weightpercent SiO₂, 9.4 weight percent Na₂ O, 61.5 weight percent H₂ O) and94.3 pounds of alum (8.34 weight percent Al₂ O₃, 24.07 weight percent H₂SO₄, 67.59 weight percent H₂ O). After completion of the addition, aone-liter portion of the slurry was removed and combined with 5.94pounds of tetramethylammonium bromide. The TMABr-containing compositionwas then returned to the bulk of the slurry to form the completereaction mixture. Digestion and crystallization was carried out at 125°C. for 45 hours. The crystalline zeolite Ω product was recovered byfiltration and was washed with water until the pH of the wash water wasbelow 11. The bulk Si/Al₂ ratio of the product was 6.8 and the valuesfor the unit cell parameters a_(o) and c_(o) were 18.21 and 7.63,respectively.

EXAMPLE 2

(a) A portion of the zeolite Omega prepared in Example 1 was dried inair at 100° C. and then calcined in air at a temperature of 540° C. for1.5 hours. The surface area, adsorption capacity for SF₆ and O₂ and theButane Cracking Activity for this calcined composition were determined.The values are reported in Table C, below.

(b) A portion of the zeolite Omega prepared in Example 1 was calcined asin part (a) supra and then subjected to ammonium ion-exchange using anaqueous solution of ammonium nitrate. The exchange was carried out inthree steps, each using 6.5 ml. of fresh ammonium nitrate solution pergram of zeolite. The exchanges were carried out at 93° C. and contactbetween zeolite and exchange medium was maintained for 60 minutes foreach of the three steps. The exchanged product was washed with distilledwater and dried in air at 100° C. Surface area, butane cracking activityand SF₆ and O₂ adsorption capacities were determined. The results arereported in Table C, below.

(c) A portion of the same calcined zeolite Omega as employed in part (b)supra was subjected to two ammonium ion exchanges using an aqueousammonium nitrate solution adjusted with nitric acid to an initial pH of0.85. Each contact between zeolite and solution was for a period of 60minutes and a temperature of 93° C. was used. For each gram of zeolite,6.5 ml. of ammonium nitrate solution was employed in each of the twosteps. The pH of the ion-exchange medium in contact with the zeolite wasat all times maintained at about 0.85 by continuous addition of nitricacid. The results of the analytical procedures performed on the productzeolite are reported in Table C, below.

(d) In a process embodiment of the present invention, a sample of thesame zeolite Omega sample as used in the foregoing parts of this exampleand ion-exchange as reported in part (b) was steamed for 2 hours at 550°C. using 100% steam. The steamed product was then again subjected to anammonium ion exchange using the same procedure and ammonium nitratesolution (pH=0.85) as in part (c). The analytical results for thiscomposition are reported in Table C, below.

EXAMPLE 3 Comparison Example

Using the same zeolite Omega starting material, the same calcination inair, the same initial ammonium ion exchange procedure and the samesteaming procedure at 550° C. as in part (d) of Example 2, supra, theproduct zeolites of those treatments were contacted and washed with a 6Naqueous HCl solution using about 2700 ml. of the acid per 100 grams ofzeolite. Contact of the zeolite with the acid was maintained for aboutone hours at a temperature of 93° C. The acid-treated product wasanalyzed for surface area, SF₆ and O₂ adsorption and Butane CrackingActivity. The results are set forth in Table C, below.

EXAMPLE 4 Comparison Example

The procedure of part (d) of Example 2, supra, was repeated using thesame starting zeolite Omega and the same ammonium ion-exchange media,but conducting the steaming step at 400° C. instead of 500° C. as in theprior example. The analytical results obtained for the product zeoliteare set forth in Table C, below.

EXAMPLE 5 Comparison Example

The procedure of Example 3, supra, was repeated using the same startingzeolite Omega, the same ion-exchange medium and the same HCl solution,but conducting the steaming step at 400° C. instead of 550° C. as in theprior example. The analytical results obtained for the product zeoliteare set forth in Table C below.

EXAMPLE 6 Comparison Example

In a particular embodiment of the process of the present invention, theprocedure of Example 2 (d) was repeated except that the steamingtemperature was increased to 700° C. The analytical results for theproduct are set forth in Table C below.

EXAMPLE 7 Comparison Example

The procedure of Example 3, supra, was repeated using the same startingzeolite Omega, the same ion-exchange medium and the same HCl solution,but conducting the steaming step at 700° C. instead of 550° C. as in theprior example. The analytical results obtained for the product zeoliteare set forth in Table C, below.

                                      TABLE C                                     __________________________________________________________________________                        Butane                 Unit Cell                                    SF.sub.6                                                                           O.sub.2                                                                            Cracking                                                                           Bulk                                                                              Framework                                                                              X-ray                                                                              Parameters,                                                                           N.sub.2                                                                             H.sub.2 O            S.A.      Capacity                                                                           Capacity                                                                           Activity                                                                           SiO.sub.2 /                                                                       IR, cm.sup.-1                                                                          Crystal-                                                                           Angstroms                                                                             Adsorption,                                                                         Adsorption           Example #                                                                           m.sup.2 /g                                                                        (Wt. %)                                                                            (Wt. %)                                                                            (K.sub.a)                                                                          Al.sub.2 O.sub.3                                                                  Symm                                                                              Asymm                                                                              linity*                                                                            a.sub.o                                                                           c.sub.o                                                                           Wt. %**                                                                             Wt.                  __________________________________________________________________________                                                             %                    1     --  --   --   --   6.8 815 1038 67956                                                                              18.21                                                                             7.63                                                                              --    --                     2(a)                                                                              148 2.9  13.1 0.083                                                                              6.8 821 1046 51414        4.0   15.5                   2(b)                                                                              338 1.9  19.3 43   6.8 816 1042 55444        10.3  16.8                   2(c)                                                                              261 1.7  12.9 0.12 24.3                                                                              802 1068 19172        8.0   6.4                    2(d)                                                                              513 7.6  20.7 237  11.9                                                                              833 1070 45394                                                                              18.21                                                                             7.59                                                                              13.3  15.1                 3     230  0.58                                                                              11.2 0.19 654 942 1087 No peaks                                                                           Amorphous                                                                             7.3   5.7                  4     396 4.1  18.9 33.26                                                                              9.6 818 1041 45391        11.3  13.9                 5     250  0.64                                                                              11.9 0    911 942 1090 No peaks     7.3   8.8                  6     550 8.5  22.0 189  31  848 1083 45811                                                                              17.99                                                                             7.51                                                                              16.5  10.9                 7     558 8.8  22.2 96   71  849 1084 43232                                                                              18.06                                                                             7.52                                                                              16.7  10.3                 __________________________________________________________________________     *Sum of five selected peaks (at d spacings 9.1 ± 0.2, 6.9 ± 0.2,        3.79 ± 0.1, 3.62 ± 0.05, 2.92 ± 0.05)                                **Measured at P/P.sub.o = 0.19                                           

The data of Table C reveal a number of unexpected phenomena. Comparisonsamong Examples 1, 2(a) and 2(b) show that neither calcination in air at540° C. nor ion exchange with an ammonium ion solution of normal, i.e.,unadjusted, pH changes the bulk Si/Al₂ ratio of the zeolite. Theconventional ammonium ion exchange of the air calcined form of thezeolite does show an increase in surface area, oxygen adsorptioncapacity and Butane Cracking Activity, the latter being a generalindicator for zeolitic acidity. The adsorption capacity for SF₆,however, is decreased by the conventional high pH ammonium exchangetreatment.

In the case of zeolite Omega at least, the implications of the capacityto adsorb SF₆ are significantly different from those arising from thecapacity to adsorb oxygen. The kinetic diameter, calculated from theminimum equilibrium cross-sectional diameter, of SF₆ is 5.5 Angstromsand for O₂ the value is 3.46. In view of the fact that all species ofhydrocarbon molecules have kinetic diameter larger than 3.46, not all ofthe internal surface area of the zeolite crystal which can be contactedby oxygen molecules can be contacted by adsorbed hydrocarbon species.The availability of internal surface area available to SF₆, however, iseffectively available to a considerable number of hydrocarbon and otherorganic molecular species. Accordingly, the catalytic effectiveness ofzeolite Omega crystals which have high SF₆ adsorptive capacity ispotentially much higher than for those forms of the zeolite having lowSF₆ capacity. As a corollary, oxygen adsorptive capacity alone is not agood measure of catalytic effectiveness.

Thus, the effect of a conventional ammonium ion exchange in lowering theSF₆ capacity of zeolite Omega as shown in Example 2(b) tends to impairthe catalytic activity of the composition, although the overallimprovement in the surface area and in the oxygen capacity, and theremoval of sodium cations results in an improvement in the ButaneCracking Activity of the composition of Example 2(b) compared with thatof Example 2(a).

The data for Example 2(c) show that there is a significant differencebetween the ion-exchange of the air calcined zeolite Omega using aconventional ammonium salt solution and one in which the pH has beenadjusted to below 4.0. Compared with Example 2(b), the Example 2(c)composition is lower in B.E.T. surface area, SF₆ capacity, oxygencapacity and Butane Cracking Activity. only the bulk Si/Al₂ ratio isincreased. This clearly demonstrates that aluminum removal alone doesnot necessarily result in an improved composition.

EXAMPLE 8

In determining the Butane Cracking Activity (K_(a)) of the modifiedzeolite Ω composition of Example 2(d), above, the composition wasactivated in flowing helium at 500° C. for one hour, and the crackingreaction carried out at 500° C. also. The results of the analysis of thecracked product effluent from the reactor after 10 minutes on streamwere as follows:

    ______________________________________                                               Constituent                                                                           Mole %                                                         ______________________________________                                               Methane 22.4                                                                  Ethane  11.1                                                                  Ethylene                                                                              12.5                                                                  Propane 49.6                                                                  Propylene                                                                             3.0                                                                   Isobutane                                                                             1.4                                                            ______________________________________                                    

The high activity of this zeolite composition (K_(a) =237) and theconversion product species indicate that the materials of this inventioncan be advantageously employed as catalysts or catalyst bases inhydrocarbon conversion reactions generally. The novel zeolites of thisinvention can be compounded into a porous inorganic matrix such assilica-alumina, silica-magnesia, silica-zirconia,silica-aluminia-thoria, silica-alumina-magnesia and the like. Therelative proportions of finely divided zeolite and inorganic matrix canvary widely with the zeolite content ranging from about 1 to 90 percentby weight, preferably from about 2 to about 50 percent by weight.

Among the hydrocarbon conversion reactions catalyzed by these newcompositions are cracking, hydrocracking, alkylation of both thearomatic and isoparaffin types, isomerization including xyleneisomerization, polymerization, reforming, oxygenate synthesis,hydrogenation, dehydrogenation, transalkylation and dealkylation, andcatalytic dewaxing.

Using these zeolite catalyst compositions which contain a hydrogenationpromoter such as platinum or palladium, heavy petroleum residual stocks,cyclic stocks and other hydrocrackable charge stocks can be hydrocrackedat temperatures in the range of 400° F. to 825° F. using molar ratios ofhydrogen to hydrocarbon in the range of between 2 and 80, pressuresbetween 10 and 3500 p.s.i.g., and a liquid hourly space velocity (LHSV)of from 0.1 to 20, preferably 1.0 to 10.

The catalyst compositions employed in hydrocracking are also suitablefor use in reforming processes in which the hydrocarbon feedstockscontact the catalyst at temperatures of from about 700° F. to 1000° F.,hydrogen pressures of from 100 to 500 p.s.i.g., LHSV values in the rangeof 0.1 to 10 and hydrogen to hydrocarbon molar ratios in the range of 1to 20, preferably between 4 and 12.

These same catalysts, i.e., those containing hydrogenation promoters,are also useful in hydroisomerization processes in which feedstocks suchas normal paraffins are converted to saturated branched chain isomers.Hydrdoisomerization is carried out at a temperature of from about 200°F. to 600° F., preferably 300° F. to 550° F. with an LHSV value of fromabout 0.2 to 1.0. Hydrogen is supplied to the reactor in admixture withthe hydrocarbon feedstock in molar proportions (H/Hc) of between 1 and5.

At somewhat higher temperatures, i.e., from about 650° F. to 1000° F.,preferably 850° F. to 950° F. and usually at somewhat lower pressureswithin the range of about 15 to 50 p.s.i.g., the same catalystcompositions are used to hydroisomerize normal paraffins. Preferably theparaffin feedstock comprises normal paraffins having a carbon numberrange of C₇ -C₂₀. Contact time between the feedstock and the catalyst isgenerally relatively short to avoid undesirable side reactions such asolefin polymerization and paraffin cracking. LHSV values in the range of0.1 to 10, preferably 1.0 to 6.0, are suitable.

The increase in the molar SiO₂ /Al₂ O₃ ratios of the present zeolitecompositions favor their use as catalysts in the conversion ofalkylaromatic compounds, particularly the catlytic disproportionation oftoluene, ethylene, trimethylbenzenes, tetramethylbenzenes and the like.In the disproportionation process isomerization and transalkylation canalso occur. Advantageously the catalyst form employed contains less than1.0 weight percent sodium as Na₂ O and is principally in the so-calledhydrogen cation or decationized form. Group VIII noble metal adjuventsalone or in conjunction with Group VI-B metals such as tungsten,molybdenum and chromium are preferably included in the catalystcomposition in amounts of from about 3 to 15 weight percent of theoverall composition. Extraneous hydrogen can, but need not, be presentin the reaction zone which is maintained at a temperature of from about400° to 750° F., pressures in the range of 100 to 2000 p.s.i.g. and LHSVvalues in the range of 0.1 to 15.

Catalytic cracking processes are preferably carried out using thosezeolites of this invention which have SiO₂ /Al₂ O₃ molar ratios of 8 to12, less than 1.0 weight percent Na₂ O and feedstocks such as gas oils,heavy naphthas, deasphalted crude oil residua, etc., with gasoline beingthe principal desired product. The decationized form of the zeoliteand/or polyvalent metal cationic form are advantageously employed.Temperature conditions of 850° to 1100° F., LHSV values of 0.5 to 10 andpressure conditions of from about 0 to 50 p.s.i.g. are suitable.

Dehydrocyclization reactions employing paraffinic hydrocarbonfeedstocks, preferably normal paraffins having more than 6 carbon atoms,to form benzene, xylenes, toluene and the like, are carried out usingessentially the same reaction conditions as for catalytic cracking. Thepreferred form of the zeolite employed as the catalyst is that in whichthe cations are principally metals of Group II-A and/or II-B, such ascalcium, strontium, magnesium. Group VIII non-noble metal cations canalso be employed such as cobalt and nickel.

In catalytic dealkylation wherein it is desired to cleave paraffinicside chains from aromatic nuclei without substantially hydrogenating thering structure, relatively high temperatures in the range of about800°-1000° F. are employed at moderate hydrogen pressures of about300-1000 p.s.i.g., other conditions being similar to those describedabove for catalytic hydrocracking. Preferred catalysts are of therelatively non-acidic type described above in connection with catalyticdehydrocyclization. Particularly desirable dealkylation reactionscontemplated herein include the conversion of methylnaphthalene tonaphthalene and toluene and/or xylenes to benzene.

In catalytic hydrofining, the primary, but not the only, objective is topromote the selective hydrodecomposition of organic sulfur and/ornitrogen compounds in the feed, without substantially affectinghydrocarbon molecules therein. For this purpose it is preferred toemploy the same general conditions described above for catalytichydrocracking, and catalysts of the same general nature described inconnection with dehydrocyclization operations. Feedstocks includegasoline fractions, kerosenes, jet fuel fractions, diesel fractions,light and heavy gas oils, deasphalted crude oil residua and the like,any of which may contain up to about 5 weight percent of sulfur and upto about 3 weight percent of nitrogen.

Similar conditions can be employed to effect hydrofining, i.e.,denitrogenation and desulfurization, of hydrocarbon feeds containingsubstantial proportions of organonitrogen and organosulfur compounds. Asobserved by D. A. Young in U.S. Pat. No. 3,783,123, it is generallyrecognized that the presence of substantial amounts of such constituentsmarkedly inhibits the activity of catalysts for hydrocracking.Consequently, it is necessary to operate at more extreme conditions whenit is desired to obtain the same degree of hydrocracking conversion perpass on a relatively nitrogenous feed than are required with a feedcontaining less organonitrogen compounds. Consequently, the conditionsunder which denitrogenation, desulfurization and/or hydrocracking can bemost expeditiously accomplished in any given situation are necessarilydetermined in view of the characteristics of the feedstocks, inparticular the concentration of organonitrogen compounds in thefeedstock. As a result of the effect of organonitrogen compounds on thehydrocracking activity of these compositions it is not at all unlikelythat the conditions most suitable for denitrogenation of a givenfeedstock having a relatively high organonitrogen content with minimalhydrocracking, e.g., less than 20 volume percent of fresh feed per pass,might be the same as those preferred for hydrocracking another feedstockhaving a lower concentration of hydrocracking inhibiting constituents,e.g., organonitrogen compounds. Consequently, it has become the practicein this art to establish the conditions under which a certain feed is tobe contacted on the basis of preliminary screening tests with thespecific catalyst and feedstocks.

Isomerization reactions are carried out under conditions similar tothose described above for reforming, using somewhat more acidiccatalysts. Olefins are preferably isomerized at temperatures of500°-900° F., while paraffins, naphthenes and alkyl aromatics areisomerized at temperatures of 700°-1000° F. Particularly desirableisomerization reactions contemplated herein include the conversion ofn-heptane and/or n-octane to isoheptanes, iso-octanes, butane toiso-butane, methylcyclopentane to cyclohexane, meta-xylene and/orortho-xylene to paraxylene, 1-butene to 2-butene and/or isobutene,n-hexene to isohexene, cyclohexene to methyl-cyclopentene, etc. Thepreferred cation form of the zeolite catalyst is that in which the ionexchange capacity is about 50-60 percent occupied by polyvalent metalssuch as Group II-A, Group II-B and rare earth metals, and 5 to 30percent of the cation sites are either decationized or occupied byhydrogen cations.

For alkylation and dealkylation processes the polyvalent metal cationform of the zeolite catalyst is preferred with less than 10 equivalentpercent of the cations being alkali metal. When employed fordealkylation of alkyl aromatics, the temperature is usually at least350° F. and ranges up to a temperature at which substantial cracking ofthe feedstock or conversion products occurs, generally up to about 700°F. The temperature is preferably at least 450° F. and not greater thanthe critical temperature of the compound undergoing dealkylation.Pressure conditions are applied to retain at least the aromatic feed inthe liquid state. For alkylation the temperature can be as low as 250°F., but is preferably at least 350° F. In alkylating benzene, tolueneand xylene, the preferred alkylating agents are olefins such as ethyleneand propylene.

What is claimed is:
 1. Zeolite composition having a chemical compositionin the anhydrous state expressed in terms of molar oxide ratios as

    a M.sub.2/n O:Al.sub.2 O.sub.3 :b SiO.sub.2

wherein "a" has a value of from about zero to about 1.2, "M" representsa cation having the valence of "n," and "b" has a value of at least 7,an x-ray diffraction pattern having at least the following d-spacings

    ______________________________________                                        d,(A)        Relative Intensity                                               ______________________________________                                        9.1 ± 0.2 VS                                                               7.9 ± 0.2 M                                                                6.9 ± 0.2 M-S                                                              5.95 ± 0.1                                                                              M-S                                                              4.69 ± 0.1                                                                              M-S                                                              3.79 ± 0.1                                                                              S                                                                3.62 ± 0.05                                                                             M-S                                                              3.51 ± 0.05                                                                             M-S                                                              3.14 ± 0.05                                                                             M-S                                                              3.08 ± 0.05                                                                             M                                                                3.03 ± 0.05                                                                             M                                                                2.92 ± 0.05                                                                             M-S                                                              ______________________________________                                    

a B-E-T nitrogen surface area of at least 500 m₂ /g, an adsorptioncapacity for SF₆ of at least 6.0 weight percent when measured at 22° C.and an SF₆ pressure of 400 mm. Hg, an adsorption capacity for oxygen ofat least 20 weight percent when measured at -183° C. and an oxygenpressure of 100 mm. Hg.
 2. Zeolite composition according to claim 1wherein the adsorption capacity for SF₆ is at least 7 weight percent andthe composition has a Butane Cracking Activity, K_(a), of at least 160.3. Zeolite composition according to claim 1 wherein "b" has a value offrom 10 to about
 190. 4. Process for preparing a zeolite composition ofclaim 1 which comprises the steps of:(a) providing an as-synthesizedzeolite Omega starting material containing alkali metal and organiccations, calcining the starting composition at a temperature sufficientto decompose the organic cations; (b) contacting the calcined product ofstep (a) with an aqueous solution of non-metallic cations under cationexchange conditions to lower the alkali metal cation content to below0.1 equivalent percent; (c) calcining the ion-exchanged product of step(b) in contact with at least 3 psia steam at a temperature of from about400° C. to 800° C. for a period sufficient to reduce the a_(o) unit cellparameter; and thereafter (d) contacting the steamed product of step (c)with a sufficient amount of an aqueous solution of ammonium ions havinga pH of less than about 4.0 and for a sufficient time to increase thebulk Si/Al₂ ratio of the zeolite composition with respect to thestarting composition of step (a) and to a value of at least
 7. 5.Process according to claim 4 wherein in step (a) the calcination iscarried out in air at a temperature of from 400° C. to 600° C. 6.Process according to claim 4 wherein the zeolite Omega starting materialcontains alkali metal and tetramethylammonium cations as a result of thesynthesis procedure.
 7. Process according to claim 6 wherein in step (c)the temperature of the steam contacting the zeolite is from about 500°C. to 575° C.
 8. Process according to claim 4 wherein the aqueoussolution of non-metallic cations employed in step (b) is an aqueoussolution of ammonium cations.
 9. Process according to claim 8 wherein instep (c) the ion-exchange product of step (b) is calcined in contactwith 100 percent steam.
 10. Process according to claim 9 wherein thecalcination in contact with 100 percent steam is for a period sufficientto reduce the unit cell parameter a_(o) to not more than about 18.21Angstroms.